Preparation of a Molecularly Imprinted Polymer Containing Europium(III)

Ions for Luminescent Sensing

Hyungwoo Kim, Youngdo Kim, Ji Young Chang

Department of Materials Science and Engineering, College of Engineering, Seoul National University, Seoul 151-744, Korea

Correspondence to: J. Y. Chang (E-mail: jichang@snu.ac.kr)

Received 3 May 2012; revised 21 July 2012; accepted 28 July 2012; published online

DOI: 10.1002/pola.26314

KEYWORDS: lanthanides; luminescence; molecular imprinting; sensors

INTRODUCTION The lanthanide(III) ions have attracted greatinterests in sensor applications due to their unique photo-physical properties. Their luminescence resulting from intra-configurational f-f transition is narrow, long-lasting, and sen-sitive to the coordination environments, which makes them apromising candidate for signal transducers. Since the lantha-nide(III) ions have very low molar absorptivities, it is diffi-cult to populate their resonance levels by direct excitation.As an alternative, indirect excitation of the lanthanide(III)ions via an organic-based antenna is widely used to obtainstrong luminescence. The organic ligands located around thelanthanide(III) ions absorb light and sensitize the lanthani-de(III) ions by intramolecular energy transfer.1

Molecular imprinting is a practical method for preparingartificial receptors. In the molecular imprinting process, acomplex of a functional vinyl monomer and a template iscopolymerized with a crosslinking agent and subsequentremoval of the template from the polymerized matrix gener-ates binding cavities.2–8 In sensor applications of molecularlyimprinted polymers, the bound molecules are usually ana-lyzed by an indirect method such as choromatography.2,3

However, this method has a drawback such that an addi-tional separation process is required. Several direct sensingmethods have been developed, involving the detection ofchanges in optical properties upon molecular binding9 byusing quantum dots,10,11 dyes,12,13 and fluorescent poly-mers14 as probes. The use of the lanthanide(III) ions to sig-nal the detection has also been reported.15–19 In these stud-ies, the sensors were designed to detect organophosphonatesand organophosphates, which have strong binding affinityfor europium(III) ions. The imprinted polymer matrices con-tained both the europium(III) ions and the sensitizing chro-mophore moieties. Once the target molecules were bound tothe ions by coordination, there was observed an increase ofluminescence intensity, regardless of their structures. This

result was attributed to the fact that the luminescence of theeuropium(III) ions is sensitive to the coordination numberand geometry as well as the ligand structure.

In this work, we report a facile synthesis of a luminescencesensor based on the europium(III) ion-containing molecu-larly imprinted polymer. Different from previous works, thisstudy uses nonchromophoric monomers for coordinationwith the europium(III) ions and for polymerization. As aresult, the europium(III) ion-containing imprinted polymershowed significant luminescence only after binding of chemi-cals bearing sensitizing chromophore moieties. This type ofsensor may be suitable for detecting chromophoric toxicchemicals, such as aromatic acid herbicides and insecticides.We chose picloram as a model template. Picloram is a widelyused chlorinated herbicide persistent in water or soil, whichis a suspicious endocrine disruptor and also a detriment tothe environment.20

EXPERIMENTAL

Materials3-Allylpentane-2,4-dione was synthesized according to thereported procedure.21 Azobisisobutyronitrile (AIBN) wasrecrystallized in methanol before use. Ethylene glycol dime-thacrylate (EGDMA) was purified by passing through acolumn filled with aluminum oxide (Aldrich) to remove theinhibitor. All other chemicals and reagent grade solventswere purchased from Aldrich and used without any furtherpurification.

Measurements1H NMR spectra were recorded on a Bruker Avance 300spectrometer (300 MHz). UV–vis spectra were obtained withthe use of a Sinco S-3150 spectrometer. Fluorescence meas-urements were performed on a Shimadzu RF-5301PC spec-trofluorometer. The pH values of solutions were measuredusing a Schott Lab-860 pH meter.

VC 2012 Wiley Periodicals, Inc.

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Polymerization of the Europium(III) ComplexTo a solution of 3-allylpentane-2,4-dione (578 mg; 4.12mmol) in ethanol (100 mL) was added a solution of NaOH(164.89 mg; 4.12 mmol) in water (5 mL). Then, solutions ofeuropium(III) chloride hexahydrate (503.60 mg; 1.37 mmol)in ethanol (2 mL) and picloram (331.87 mg; 1.37 mmol) inethanol (2 mL) were added dropwise in sequence. After stir-ring 2 h at room temperature, EGDMA (3.89 mL; 20.62mmol) and AIBN (225.69 mg; 1.37 mmol) were added andthe reaction mixture was refluxed for 4 h. The white precipi-tates were isolated by filtration, washed with tetrahydrofu-ran, water, and methanol, and dried in vacuo.

Extraction of the Europium(III) Ions and the TemplateMoleculesThe crosslinked polymers were stirred overnight in a HCland ethanol solution (1:9, v/v) and isolated by filtration. Thepolymers were soxhlet extracted with ethanol for 2 days,collected, and dried in vacuo.

Incorporation of the Europium(III) Ion into thePolymer Matrix (MIP-Eu)To a mixture of the extracted polymer (1.20 g) in ethanol(100 mL) and a solution of NaOH (81.86 mg; 2.05 mmol) inwater (2 mL) was added a solution of europium(III) chloridehexahydrate (250.00 mg; 0.68 mmol) in ethanol (2 mL). Thereaction mixture was refluxed overnight. The white precipi-tates were isolated by filtration, washed with water andmethanol, and dried in vacuo.

Preparation of the Europium(III) CoordinatedNonimprinted Polymer (NIP-Eu)This polymer was prepared in the same manner as MIP-Euexcept that picloram was omitted.

Rebinding and Selectivity TestIn rebinding test, picloram was dissolved in methanol (10mL) at various concentrations (0, 0.05, 0.1, 0.2, 0.3, and 0.4mM). Then, the pH of each solution was adjusted to 6 with a0.1 M aqueous NaOH solution. MIP-Eu or NIP-Eu (100 mg)was added to the solutions. After incubation for 1 h, lumi-nescence at 616 nm was monitored. In selectivity test,picloram and its analogues were dissolved in methanol (10mL) at a concentration of 0.4 mM and the pH of each solu-tion was adjusted to 6 with a 0.1 M aqueous NaOH solution.MIP-Eu or NIP-Eu (100 mg) was added to solutions. Afterincubation for 1 h, luminescence at 616 nm was monitored.All measurements were repeated three times.

RESULTS AND DISCUSSION

Scheme 1 shows the fabrication of the europium(III) ion-con-taining molecularly imprinted polymer. We used 3-allylpen-tane-2,4-dione and picloram (4-amino-3,5,6-trichloro-2-pyri-dinecarboxylic acid, template) as ligands for the complexationwith the europium(III) ion. 3-Allylpentane-2,4-dione has ab-diketone structure and an allyl group. b-Diketone is wellknown for its good coordinating ability and widely used as aligand in metal coordination.22 Three b-diketone ligands werefirst coordinated with the europium(III) ion by the reaction of

SCHEME 1 Schematic description of the preparation of the europium(III) ion-containing molecularly imprinted polymer.

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3-allylpentane-2,4-dione and europium(III) chloride hexahy-drate in basic aqueous ethanol. Generally, the coordinationnumber of a trivalent europium ion complex is eight or nine.We replaced the chloride or water molecules occupying theremaining two exchangeable coordination sites with picrolam.Picloram has an aromatic ring, which can function as a sensi-tizing chromophore and two coordination sites, pyridinicnitrogen and carboxylic oxygen.

Figure 1 shows the changes in emission of the b-diketonecoordinated europium(III) ion by the addition of picloram atpH 6 under excitation at 320 nm. The emission intensityincreased continuously until one equivalent of picrolam wasadded to the europium(III) ion, suggesting the formation of a

europium(III) ion complex coordinated with three b-dike-tones and one picrolam.

Without isolation of the complex, we carried out the copoly-merization of its allylic groups with EGDMA using AIBN as aninitiator. The radical polymerization of allyl monomers gener-ally produces oligomers because of the chain transfer to themonomers,23–26 but they are copolymerized with acrylates.27,28

We used a considerably large amount of initiator to obtain acrosslinked, insoluble polymer as white fine powders.

The template molecules together with the europium(III) ionswere removed from the polymer matrix by solvent extractionwith HCl/ethanol, and then only the europium(III) ions werereintroduced into the polymer matrix by reacting the poly-mer with europium(III) chloride hexahydrate. By thisprocess, the europium(III) ions were placed in the bindingcavities of the imprinted polymer (MIP-Eu) through coordi-nation with three b-diketo groups. A nonimprinted polymer(NIP-Eu) was also prepared in the same manner as MIP-Euexcept that picloram was omitted.

The ability of MIP-Eu to recognize the template (picrolam)was investigated by photoluminescence spectroscopy. For therebinding test, picloram solutions of various concentrationsin methanol were prepared and their pHs were adjusted to 6with a 0.1 M aqueous NaOH solution. MIP-Eu was added toeach solution and incubated for 1 h. The emissions of themixtures were measured at an excitation wavelength of 250nm. Figure 2(a) shows the spectra obtained by subtractingthe spectrum of the MIP-Eu in methanol from the originalspectra of the mixtures. The characteristic and narrow emis-sion peaks of the europium(III) ions appeared at 594 and616 nm, corresponding to 5D0 ! 7F1 and 5D0 ! 7F2 transi-tions, respectively. The intensities of these peaks increasedwith the increase in the picrolam concentration.

Figure 2(b) shows the plots of the changes in the emissionintensity of MIP-Eu and NIP-Eu versus the concentration of

FIGURE 2 (a) Emission spectra of MIP-Eu in aqueous methanol in the presence of various concentrations of picloram. Emission

spectra were obtained at 250 nm excitation. (b) The concentration effect of picloram on emissions of MIP-Eu and NIP-Eu. DI: emis-

sion intensity change upon addition of picloram (I � I0), I0: emission intensity in the absence of picloram.

FIGURE 1 The changes in the emission of the b-diketone coor-

dinated europium(III) ion by addition of picloram at pH 6 under

excitation at 320 nm. The b-diketone coordinated europium(III)

ion was prepared from 1 eq. of europium(III) ion and 3 eq of 3-

allylpentane-2,4-dione in MeOH. The inset shows the changes

in the 616 nm emission.

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picrolam. A DI/I0 parameter was used to analyze the lumi-nescence changes, where I0 was the emission intensity in theabsence of picloram and DI was equal to I � I0. The emis-sion of MIP-Eu was much more sensitive to the picrolamconcentration than that of NIP-Eu was. The DI/I0 of MIP-Euincreased sharply with the increase in the picrolam concen-tration up to 0.1 mM and then slowly increased thereafter.At the concentration of 0.1 mM, the enhancement of theemission intensity of MIP-Eu was more than four times thatof NIP-Eu, demonstrating the high recognition ability of MIP-Eu. Because the europium(III) ions were placed in the bind-ing cavities, the template molecules were likely trapped inthe cavities via coordination with the europium(III) ions.

The binding affinity of MIP-Eu for picrolam was also con-firmed by visual observation. Figure 3 shows the photographsof the as-synthesized polymer, MIP-Eu and the picloram-bound MIP-Eu. The polymers did not have a distinct color indaylight. Under irradiation at 365 nm, however, the as-synthe-sized polymer containing the europium(III) ions and picrolamappeared bright red. The imprinted polymer (MIP-Eu) didnot show visible emission owing to the absence of a chromo-phoric ligand, but exhibited a red color after incubation for 1h in a 0.4 mM picloram solution in methanol.

The specific recognition ability of MIP-Eu was investigatedfor the template and its structural analogs, dicamba and

2-amino-4,6-dichloropyrimidine-5-carboxaldehyde (ADC) byphotoluminescence spectroscopy. The rebinding test was car-ried out in the same manner as described above. Solutionsof picloram and its analogues in methanol were prepared tohave a concentration of 0.4 mM and a pH of 6. After incuba-tion of MIP-Eu for 1 h, the emissions of the mixtures weremeasured at an excitation wavelength of 250 nm. Figure 4(a)shows the spectra obtained by subtracting the spectrum ofthe MIP-Eu in methanol from the original spectra of the mix-tures. Figure 4(b) shows the changes in the emission inten-sity (DI/I0) of MIP-Eu by addition of analytes. MIP-Eushowed the highest recognition ability for picrolam and low-est recognition ability for ADC. Although dicamba has thecarboxylic oxygen which can function as a possible coordina-tion site, its binding affinity to MIP-Eu was much lower thanthat of picrolam, proving that the template was most accessi-ble to the europium(III) ion for coordination.

CONCLUSIONS

We demonstrated that the molecularly imprinted systembearing the europium(III) ions could be used for the directdetection of the chromophoric template molecule. The euro-pium(III) ion was located in the binding cavity, which wasproduced in the nonchromophoric crosslinked polymer ma-trix. In the binding process, the template was trapped in the

FIGURE 4 (a) Emission spectra of MIP-Eu in aqueous methanol in the presence of picloram and its analogues (0.4 mM). Emission

spectra were obtained at 250 nm excitation. (b) Structures of analytes and their effect on emissions of MIP-Eu and NIP-Eu. DI:emission intensity change upon addition of picloram (I � I0), I0: emission intensity in the absence of picloram.

FIGURE 3 Photographs of the as-synthesized polymer, MIP-Eu and MIP-Eu incubated in a 0.4 mM picloram solution in methanol

for 1 h taken in daylight (left) and under 365 nm irradiation (right).

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cavity to coordinate with the europium(III) ion. Theimprinted polymer had high recognition ability for the tem-plate and showed enhanced luminescence emission in itspresence. Since the luminescence emission of the europiu-m(III) ion is very sensitive to the chromophoric ligand, thissystem has great potential to detect chromophoric chemicals.

ACKNOWLEDGMENTS

This work was supported by the National Research Foundationof Korea (NRF) grant funded by the Korea government (MEST)(No. 2010-0017552).

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